Objects behind glass appear too dark
There are several things causing this:


Lenses have no thickness (as pointed out by Jerryno)
Rendering caustics is hard


Lens geometry

In the real world the lenses are likely solid objects separate from the goggles, merely fastened into place within the goggle frames by some means.

So model virtual lenses separately and put them in the virtual goggle frames:


Start by making the goggle frames solid on their own, without any lenses:


Then model a separate lens mesh and put it in the frame. It shouldn't be connected to the frame (if you hover over it and press L, it and only it should be selected):


Ensure that the eye mesh doesn't intersect the lens:




Caustics

Even with all that, a normal glass shader will still likely look dark without lots of samples. This is due to the fact that the only way for light to get behind and illuminate the eyes is by refracting through the lenses then reflecting back out, which is a fairly unlikely path for a ray to take.  See this question for more detail.

To work around this, use a setup like the one described in How to illuminate the darkness inside glass objects in Cycles?: